High rigidity glass-ceramic substrate for a magnetic information storage medium

ABSTRACT

A high rigidity glass-ceramic substrate for a magnetic information storage medium has a ratio of Young&#39;s modulus to specific gravity within a range from 37 to 63 and comprises Al 2 O 3  within a range from 10% to less than 20%. Predominant crystal phases of the glass-ceramic substrate consist of (1) cordierite or cordierite solid solution and (2) one or more crystals selected from the group consisting of Spinel crystal, Spinel crystal solid solution, enstatite, enstatite solid solution, β-quartz and β-quartz solid solution.

BACKGROUND OF THE INVENTION

This invention relates to a glass-ceramic substrate for a magneticinformation storage medium used in an information storage device and,more particularly, to a glass-ceramic substrate for a magneticinformation storage medium such as a magnetic disk substrate having ahighly smooth substrate surface suitable for use in a near contactrecording or a contact recording used mainly in the ramp loading systemand also having a high Young's modulus and low specific gravity capableof coping with a high speed rotation of the storage medium. Theinvention relates also to a method for manufacturing this glass-ceramicsubstrate for a magnetic information storage medium. This inventionrelates further to a magnetic information storage medium made by forminga film on such glass-ceramic substrate for a magnetic informationstorage medium. In this specification, the term “magnetic informationstorage medium” includes fixed type hard disks, removable type harddisks and card type hard disks used respectively such as so-called harddisks for personal computers and other magnetic information storagemedium in the form of a disk which can be used for storage of data andcan be also used in digital video cameras and digital cameras.

In contrast to conventional fixed type magnetic information storagedevices, there have recently been proposed and put into practicemagnetic information storage device of a removable type or a card typeand developments of applications thereof for digital video cameras anddigital cameras have been started. Such tendency relates to utilizationof personal computers for multi-media purposes and prevailing of digitalvideo cameras and digital cameras and, for handling data of a large sizesuch as data of a moving image or voice, there is an increasing demandfor a magnetic information storage device of a larger storage capacity.For coping with this demand, a magnetic information storage medium isrequired to increase its bit and track density and increase the surfacerecording density by reducing the size of the bit cell. On the otherhand, as for a magnetic head, there is an increasing tendency towardadopting the near contact recording system and further the contactrecording system according to which the magnetic head operates in closerproximity to the disk surface in accordance with the reduction of thebit cell size.

Aluminum alloy has been conventionally used as a material of a magneticinformation storage medium substrate. The aluminum alloy substrate,however, tends to produce a substrate surface having projections orspot-like projections and depressions during the polishing process dueto defects inherent in the material. As a result, the aluminum alloysubstrate is not sufficient in flatness and smoothness as the abovedescribed magnetic information storage medium substrate of highrecording density. Further, since the aluminum alloy is a soft material,its Young's modulus and surface hardness are low with the result that asignificant degree of vibration takes place during rotation of the diskdrive at a high speed rotation which leads to deformation of the diskand hence it is difficult to cope with the requirement for making thedisk thinner. Furthermore, the disk tends to be deformed due to contactwith the magnetic head with resulting damage to the contents of thedisk. Thus, the aluminum alloy cannot cope sufficiently with therequirement for recording with a high recording density.

As a material for overcoming the above problems of the aluminum alloysubstrate, known in the art are chemically tempered glasses such asalumino-silicate glasses (SiO₂—Al₂O₃—Na₂O) disclosed in Japanese PatentApplication Laid-open No. Hei 8-48537 and Japanese Patent ApplicationLaid-open No. Hei 5-32431. This material, however, has the followingdisadvantages:

(1) Since polishing is made after the chemical tempering process, thechemically tempered layer is seriously instable in making the diskthinner. Further, the chemically tempered layer causes change with timeafter use for a long period of time with resulting deterioration in themagnetic property of the disk.

(2) Since the chemically tempered glass contains Na₂O and K₂O as itsessential ingredients, the film forming property of the glass isadversely affected, and a barrier coating over the entire surface of theglass becomes necessary for preventing elution of Na₂O and K₂Oingredients and this prevents stable production of the product at acompetitive cost.

(3) The chemical tempering is made for improving mechanical strength ofthe glass but this basically utilizes strengthening stress within thesurface layer and the inside layer of the glass and hence its Young'smodulus is about 83 GPa or below which is about equivalent to ordinaryamorphous glass. Therefore, use of the glass for a disk with a highspeed rotation is limited and hence it is not sufficient for a magneticinformation storage medium substrate.

Aside from the aluminum alloy substrate and chemically tempered glasssubstrate, known in the art are some glass-ceramic substrates. Forexample, Japanese Patent Application Laid-open No. Hei 9-35234 andEP0781731A1 disclose glass-ceramic substrates for a magnetic disk madeof a Li₂O—SiO₂ system composition and has crystal phases of lithiumdisilicate and β-spodumene, or crystal phases of lithium disilicate andβ-cristobalite. In these publications, however, the relation betweenYoung's modulus and specific gravity with respect to high speed rotationis not taken into consideration or suggested at all. The upper limit ofYoung's modulus of these glass-ceramics is no more than 100 GPa.

For improving the Young's modulus, Japanese Patent Application Laid-OpenNo. Hei 9-77531 discloses a glass-ceramic of a SiO₂—Al₂O₃—MgO—ZnO—TiO₂system which contains a large amount of Spinel crystal as itspredominant crystal phase and MgTi₂O₅ and several other crystals as itssubordinate crystal phases and has Young's modulus of 93.4 GPa to 160.11GPa and a substrate for a high-rigidity magnetic disk made of thisglass-ceramic. The glass-ceramic has, as its predominant crystal phase,Spinel crystal represented by (Mg/Zn) Al₂O₃ and/or (Mn/Zn)₂ TiO₄containing a large amount of Al₂O₃ and has several other optionalcrystals as its subordinate crystal phaes. This glass-ceramic issignificantly different from the glass-ceramic of the present inventionwhich, as will be described later, contains a relatively small amount ofAl₂O₃ and has a high Young's modulus and a low specific gravity.Addition of such a large amount of Al₂O₃ is undesirable from thestandpoint of production because it causes deterioration of meltingproperty of the base glass and also deterioration of resistance todevitrification. Further, in this publication, the relation of Young'smodulus (GPa)/specific gravity and the value of specific gravity per sewhich are necessary for the high speed rotation are not taken intoconsideration or suggested at all. Particularly, the specific gravity inthis publication is a high value of 2.87 or over. This publicationtherefore merely proposes a glass-ceramic substrate made of a rigidmaterial. Moreover, the glass-ceamic of this system has the seriousdisadvantage that processability is poor and therefore is not suited fora large scale production since it is too hard and hence the improvementof this glass-ceramic as a high recording density magnetic informationstorage medium substrate is still not sufficient.

It is therefore an object of the invention to eliminate the abovedescribed disadvantages of the prior art materials and provide aglass-ceramic substrate for a magnetic information storage mediumcapable of coping with the tendency toward a high recording densityrecording, namely having excellent melting property, resistance todevitrification and processability, excellent smoothness of the surfaceof the substrate which is sufficient for contact recording of a highrecording density and having a high value of Young's modulus and a lowvalue of specific gravity which are sufficient for a high speed rotationof the disk.

It is another object of the invention to provide a magnetic informationstorage medium made by forming a film of a magnetic medium on thisglass-ceramic substrate.

SUMMARY OF THE INVENTION

Accumulated studies and experiments made by the inventors of the presentinvention for achieving the above described objects of the inventionhave resulted in the finding, which has led to the present invention,that for coping with driving of the disk with a high speed rotation, amagnetic information storage medium substrate must have a high rigidityand a low specific gravity for preventing vibration of the disk causedby deflection and that an optimum ratio of Young's modulus to specificgravity of the substrate is:

Young's modulus (GPa)/specific gravity=37 to 63.

The inventors of the present invention have also found that aglass-ceramic substrate for a magnetic information storage medium can beproduced which has predominant crystal phases limited to (1) cordieriteor cordierite solid solution and (2) one or more crystals selected fromthe group consisting of Spinel crystal, Spinel crystal solid solution,enstatite, enstatite solid solution, β-quartz and β-quartz solidsolution has grown crystal grains which are of a fine globular grain,has excellent melting property, resistance to devitrification andpolishing processability, has superior flatness and smoothness in thesurface after polishing, and has a high value of Young's modulus and alow value of specific gravity capable of coping with driving of the diskwith a high speed rotation.

For achieving the above described object of the invention, there isprovided a high rigidity glass-ceramic substrate for a magneticinformation storage medium having a ratio of Young's modulus to specificgravity within a range from 37 to 63 and comprising Al₂O₃ within a rangefrom 10% to less than 20%.

In one aspect of the invention, the high rigidity glass-ceramicsubstrate is substantially free of Na₂O, K₂O, Li₂O and PbO.

In another aspect of the invention, predominant crystal phases in thesubstrate consist of (1) cordierite or cordierite solid solution and (2)one or more crystals selected from the group consisting of Spinelcrystal, Spinel crystal solid solution, enstatite, enstatite solidsolution, β-quartz and β-quartz solid solution.

In another aspect of the invention, this substrate is provided by heattreating a base glass having a composition expressed on the basis ofcomposition of oxides which consists in weight percent of:

SiO₂  40-60% MgO  10-20% Al₂O₃  10-less than 20% P₂O₅   0-4% B₂O₃   0-4%CaO 0.5-4% BaO   0-5% ZrO₂   0-5% TiO₂ 2.5-8% Sb₂O₃   0-1% As₂O₃   0-1%F   0-3% SnO₂   0-5% MoO₃   0-3% CeO   0-5% Fe₂O₃   0-5%

In another aspect of the invention, a grain diameter of the respectivepredominant crystal phases is within a range from 0.05 μm to 0.30 μm.

In another aspect of the invention, a coefficient of thermal expansionwithin a range from −50° C. to +70° C. is within a range from 30×10⁻⁷/°C. to 50×10⁻⁷/° C.

In another aspect of the invention, the surface of the substrate afterpolishing has surface roughness (Ra) of 3 to 9 Å and maximum surfaceroughness (Rmax) of 100 Å or below.

In another aspect of the invention, the high rigidity glass-ceramicsubstrate is provided by heat treating the base glass at a nucleationtemperature within a range from 650° C. to 750° C. and at acrystallization temperature within a range from 750° C. to 1050° C.

In still another aspect of the invention, there is provided a magneticinformation storage disk having a film of a magnetic medium formed onsaid high rigidity glass-ceramic substrate.

In this specification, “Spinel crystal” means at least one crystalselected from the group consisting of (Mg and/or Zn)Al₂O₄, (Mg and/orZn)₂TiO₄, and a mixture in the form of a solid solution of these twotypes of crystals. The term “solid solution” in this specification meanseach of the above described crystals a part of which is substituted byother ingredient or in which other ingredient has been mixed. The term“β-quartz solid solution” means β-quartz crystal a part of which issubstituted by an element other than Si or in which an element otherthan Si is mixed. β-quartz solid solution includes β-eucryptite(β-Li₂O·Al₂O₃·2SiO₂) in which Li and Al at the ratio of one to onepartly substitute Si, β-eucryptite solid solution which is β-eucryptitecrystal a part of which is substituted by other element or in whichother element has been mixed, and a mixture thereof.

DETAILED DESCRIPTION OF THE INVENTION

Reasons for limiting the physical properties, predominant crystal phasesand crystal grain diameter, surface characteristics and composition ofthe glass-ceramics of the present invention will now be described. Thecomposition of the glass-ceramics is expressed on the basis ofcomposition of oxides as in their base glass.

Young's modulus and specific gravity will first be described. Asdescribed previously, there has been an increasing tendency towarddriving of a magnetic information storage medium with a high-speedrotation for improving the recording density and data transfer rate. Ithas been found by the inventors of the present invention that, forsufficiently conforming to such tendency, a material for a magneticinformation storage medium must have high rigidity and low specificgravity in order to prevent vibration caused by deflection of the diskwhich occurs during high speed rotation of the disk, e.g., 10,000rev/min. or over. A disk which has high rigidity but high specificgravity causes deflection during a high speed rotation with resultingvibration of the disk. It has been found that an apparentlycontradictory balance of physical properties of the material must beadopted, namely high rigidity and low specific gravity and this balancehas been found to exist in the ratio of Young's modulus (GPa) tospecific gravity within a range from 37 to 63. A preferable range ofYoung's modulus(GPa)/specific gravity is 40 to 63, a more preferablerange thereof is 47 to 63 and the most preferable range thereof is 50 to63. It has been also found that there is a preferable range of rigidity,That is, from the standpoint of preventing vibration, the materialshould preferably have rigidity of at least 120 GPa even if it has lowspecific gravity but, having regard to processability and increase ofspecific gravity, the upper limit of rigidity should preferably be 150GPa. The same applies to specific gravity. That is, from the standpointof preventing vibration, the material should preferably have specificgravity of 3.50 or less even if it has high rigidity, because otherwisevibration tends to occur during a high speed rotation due to its weight,but the material should preferably have specific gravity of 2.3 or overbecause otherwise it is difficult to obtain a substrate having desiredrigidity. A more preferable range of specific gravity from thesestandpoints is 2.5 to 3.3.

If a material for a substrate contains Na₂O, K₂O or Li₂O, Na ion, K ionor Li ion of these ingredient diffuses into a magnetic film during thefilm forming process when the temperature of the substrate rises to ahigh temperature (this phenomenon is particularly remarkable in a bariumferrite perpendicular magnetic film) and thereby causes abnormal growthof grains of the magnetic film and deterioration of orientation of themagnetic film. It is therefore important that the material of thesubstrate is substantially free of such ingredients. The material of thesubstrate should also be substantially free of PbO which is notdesirable because of the environmental problems.

Description of predominant crystal phases of the substrate of theinvention will now be described. In one aspect of the invention,predominant crystal phases consist of (1) cordierite or cordierite solidsolution and (2) one or more crystals selected from the group consistingof Spinel crystal, Spinel crystal solid solution, enstatite, enstatitesolid solution, β-quartz and β-quartz solid solution. This is becausethese crystal phases are advantageous in that they have excellentprocessability, contribute to increase in rigidity, can have arelatively small grain diameter of grown crystal, and can realize a muchlower specific gravity than other crystal phases. Growth and contentratio of crystal phases of cordierite, Spinel crystal, enstatite andβ-quartz are determined by the content ratio of MgO, SiO₂, and Al₂O₃.Growth and content ratio of these four crystal phases and solid solutionof these four crystal phases are determined by the content ratio Of MgO,SiO₂, Al₂O₃ and other ingredients.

Description will now be made about crystal grain diameter of growncrystal and surface roughness. As was previously described, for copingwith the near contact recording system and the contact recording systemfor improving the recording density, the smoothness of the surface ofthe magnetic information storage medium must be improved over theconventional magnetic medium. If one attempts to carry out high densityinputting and outputting of information on the magnetic medium havingthe conventional level of smoothness, the distance between the magnetichead and the surface of the magnetic medium is so large that inputtingand outputting of magnetic signals cannot be achieved. If one attemptsto reduce this distance, the magnetic head will collide with projectionsof the magnetic medium resulting in damage of the magnetic head ormagnetic medium. For coping with the near contact recording system andthe contact recording system, it has been found that the substrateshould have smoothness expressed in terms of surface roughness (Ra) of 3to 9 Å and maximum surface roughness (Rmax) of 100 Å or below.Preferably, the surface roughness (Ra) should be 3 to 7 Å and themaximum surface roughness (Rmax) should be 95 Å or below. Morepreferably, the surface roughness (Ra) should be 3 to 6 Å and themaximum surface roughness (Rmax) should be 90 Å or below.

For obtaining a glass-ceramic substrate having a high rigidity and highflatness (3-9 Å in the data area) as in the present invention, thecrystal grain diameter and crystal grain shape are important factors. Ithas been found that a substrate having a grain diameter which is largeror smaller than the range of diameter defined in the claims cannotachieve the desired strength and surface roughness.

In increasing the bit number and track density and reducing the size ofthe bit cell, difference in coefficient of thermal expansion between themagnetic film and the substrate significantly affects achievements ofthese objects. For this reason, it has been found that the coefficientof thermal expansion within a temperature range from −50° C. to +70° C.should preferably be within a range from 30×10−7/° C. to 50×10⁻⁷/° C.

In the glass-ceramic substrate according to the invention, the abovedescribed content ranges of the respective ingredients have beenselected for the reasons stated below.

The SiO₂ ingredient is a very important ingredient which, by heating abase glass, forms cordierite, cordierite solid solution, enstatite,enstatite solid solution, β-quartz and β-quartz solid solution crystalsas predominant crystal phases. If the amount of this ingredient is below40%, the crystal phases grown in the glass-ceramic are instable andtheir texture tends to become too rough whereas if the amount of thisingredient exceeds 60%, difficulty arises in melting and forming thebase glass. For the growth of the crystal phases, conditions of heattreatment are also important factors. A preferable range of thisingredient which enables a broadened heat treatment conditions is48.-58.5%.

The MgO ingredient also is a very important ingredient which, by heatinga base glass, forms cordierite, cordierite solid solution, Spinelcrystal, Spinel crystal solid solution, enstatite, enstatite solidsolution, β-quartz and β-quartz solid solution as predominant crystalphases. If the amount of this ingredient is below 10%, desired crystalscannot be obtained and the grown crystals of the obtained glass-ceramicare instable, and their texture is too rough and melting propertydeteriorates. If the amount of this ingredient exceeds 20%, resistanceto devitrification is reduced. A preferable range of this ingredient is13-20% for the same reason as in the SiO₂ ingredient.

The Al₂O₃ ingredient is a very important ingredient which, by heating ofa base glass, forms cordierite, cordierite solid solution, Spinelcrystal, Spinel crystal solid solution, β-quartz and β-quartz solidsolution as predominant crystal phases. If the amount of this ingredientis below 10%, desired crystals cannot be obtained and the grown crystalsof the obtained glass-ceramic are instable and their texture is toorough and melting property deteriorates. If the amount of thisingredient is 20% or over, melting property of the base glass andresistance to devitrification are deteriorated and, moreover, the amountof Spinel crystal increases excessively with the result that hardness ofthe substrate becomes so large that processability in processes such aspolishing is reduced significantly. For the same reason as in the aboveingredients, a preferable range of this ingredient is 10 to less than18% and a more preferable range is 10-17%.

The P₂O₅ ingredient functions as a nucleating agent for the glass andalso is an ingredient which is effective for improving melting andmolding properties of the base glass and for improving resistance todevitification. The amount of this ingredient up to 4% will suffice forthese purposes. A preferable range of this ingredient is 1-3%.

The B₂O₃ ingredient is effective for controlling viscosity duringmelting and forming of the base glass. The amount of this ingredient upto 4% will suffice.

The CaO ingredient is an ingredient which improves melting property ofthe base glass and prevents grown crystals from becoming too rough. Ifthe amount of this ingredient is below 0.5%, these effects cannot beobtained whereas if the amount of this ingredient exceeds 4%, the growncrystals become too rough, the crystal phases undergo change andchemical durability of the glass-ceramic deteriorates. A preferablerange of this ingredient is 1-3%.

The BaO ingredient may be add for improving melting property of theglass. The amount of this ingredient up to 5% will suffice. A preferablerange of this ingredient is 1-3%.

The ZrO₂ and TiO₂ ingredients are very important ingredients whichfunction as a nucleating agent for the glass and also are effective formaking the grown crystal grains finer and improving mechanical strengthand chemical durability of the material. The amount of the ZrO₂ingredient up to 5% will suffice. As for the TiO₂ ingredient, if theamount of this ingredient is below 2.5%, the above effects cannot beobtained whereas if the amount of this ingredient exceeds 8%, difficultyarises in melting of the base glass and resistance to devitrificationdeteriorates. For the same reason as in the SiO₂ ingredient, apreferable range of this ingredient is 2-8%.

The Sb₂O₃ and/or As₂O₃ ingredients may be added as refining agents inmelting the glass. It will suffice if each ingredient is added up to 1%.

The F ingredient may be added for improving melting property of theglass. It will suffice if this ingredient is added up to 3%.

The SnO₂, MoO₃, CeO and Fe₂O₃ ingredients may be added up to the totalamount of 5% as a coloring agent or for improving sensitivity ofdetecting defects on the surface of the substrate by coloring thesubstrate and also improving laser absorption characteristics for an LDexcited laser. As to MoO₃, it will suffice if the the MoO₃ ingredient upto 3% is added. The SnO₂ and MoO₃ ingredients are important in that theyhave light transmissivity in the glass state before the heat treatmentand have the coloring property after the crystallization processing.

For manufacturing the glass-ceramic substrate for a magnetic informationstorage medium according to the invention, the base glass having theabove described composition is melted, is subjected to heat formingand/or cold forming, is heat treated for producing a crystal nucleusunder a temperature within a range from 650° C. to 750° C. for about oneto twelve hours, and further is heat treated for crystallization under atemperature within a range from 750° C. to 1050° C. for about one totwelve hours.

EXAMPLES

Examples of the present invention will now be described.

Tables 1 to 7 show examples (No. 1 to No. 17) of compositions of theglass-ceramic substrate for a magnetic information storage medium madeaccording to the invention and three comparative examples, i.e., theprior art chemically tempered alumino-silicate glass (Japanese PatentApplication Laid-open No. Hei 8-48537), Comparative Example No. 1, theLi₂O—SiO₂ glass-ceramics (Japanese Patent Application Laid-open No. Hei9-35234, Comparative Example No. 2, and the SiO₂—Al₂O₃—MgO—ZnO—TiO₂glass-ceramics (Japanese Patent Application Laid-open No. Hei 9-77531),Comparative Example No. 3 together with the temperature of nucleation,temperature of crystallization, crystal phase, crystal grain diameter,Young's modulus, specific gravity, Young's modulus (GPa)/specificgravity, surface roughness (Ra) after polishing, maximum surfaceroughness (Rmax) after polishing, and coefficient of thermal expansionin the range from −50° C. to +70° C. Amounts of the respectiveingredients are expressed in weight percent.

TABLE 1 Examples 1 2 3 SiO₂ 54.0 56.8 57.5 MgO 14.0 16.0 16.0 Al₂O₃ 19.517.0 14.0 P₂O₅ — — 1.0 B₂O₃ — 1.0 2.0 CaO 2.0 2.2 2.5 BaO 2.0 — — ZrO₂ —— 0.5 TiO₂ 5.0 5.5 6.0 Sb₂O₃ — — — As₂O₃ 0.5 0.5 0.5 F — — — Fe₂O₃ 3.01.0 — Nucleation temperature (° C.) 670 700 680 Crystallizationtemperature (° C.) 1000 950 930 Predominant crystal phases cordieritecordierite cordierite and grain diameter 0.3 μm 0.3 μm 0.3 μm SpinelSpinel enstatite crystal crystal 0.1 μm 0.1 μm 0.1 μm Young's modulus(GPa) 135 128 143 Specific gravity (g/cc) 2.65 2.60 2.58 Young's modulus(GPa)/specific 50.9 49.2 55.4 gravity Surface roughness Ra (Å) 9 7 8Maximum surface roughness Rmax 98 81 78 (Å) Coefficient of thermalexpansion 37 35 46 (× 10^(−7/° C.) (−50° C. to + 70° C.))

TABLE 2 Examples 4 5 6 SiO₂ 57.1 45.6 56.0 MgO 14.0 18.0 10.4 Al₂O₃ 17.919.5 15.0 P₂O₅ — 3.0 — B₂O₃ 3.0 2.5 3.0 CaO 2.0 2.0 3.8 BaO — 0.5 — ZrO₂— — — TiO₂ 5.5 8.0 7.0 Sb₂O₃ — — — As₂O₃ 0.5 0.9 — F — — — Fe₂O₃ — — 4.8Nucleation temperature (° C.) 650 650 720 Crystallization temperature (°C.) 980 940 1000 Predominant crystal phases cordierite cordieritecordierite and grain diameter 0.3 μm 0.3 μm 0.3 μm β-quartz β-quartzβ-quartz 0.1 μm 0.1 μm 0.1 μm Young's modulus (GPa) 133 121 141 Specificgravity (g/cc) 2.64 2.50 2.90 Young's modulus (GPa)/specific 50.4 48.448.6 gravity Surface roughness Ra (Å) 5 4 9 Maximum surface roughnessRmax 64 48 94 (Å) Coefficient of thermal expansion 30 48 36 (× 10⁻⁷/°C.) (−50° C. to +70° C.)

TABLE 3 Examples 7 8 9 SiO₂ 59.5 60.0 41.7 MgO 17.0 19.5 17.0 Al₂O₃ 12.010.9 18.7 P₂O₅ 3.0 0.9 3.9 B₂O₃ 2.0 1.5 1.0 CaO 0.5 — 3.8 BaO 2.0 — —ZrO₂ — — 2.8 TiO₂ 4.0 3.5 6.6 Sb₂O₃ — 0.9 0.5 As₂O₃ — — — F — 2.8 —Fe₂O₃ — — 4.0 Nucleation temperature (° C.) 700 730 750 Crystallizationtemperature (° C.) 960 1000 1050 Predominant crystal phases cordieritecordierite cordierite and grain diameter 0.3 μm 0.3 μm 0.3 μm β-quartzβ-quartz β-quartz 0.1 μm 0.1 μm 0.1 μm Young's modulus (GPa) 138 120 151Specific gravity (g/cc) 2.81 2.60 2.65 Young's modulus (GPa)/specific49.1 46.2 57.0 gravity Surface roughness Ra (Å) 6 8 7 Maximum surfaceroughness Rmax 70 78 67 (Å) Coefficient of thermal expansion 32 41 38 (×10⁻⁷/° C.) (−50° C. to +70° C.)

TABLE 4 Examples 10 11 12 SiO₂ 45.1 55.0 56.0 MgO 18.0 18.0 11.0 Al₂O₃19.5 10.0 14.5 P₂O₅ 3.0 1.0 2.0 B₂O₃ 2.5 3.5 3.0 CaO 2.0 3.0 4.0 BaO 0.51.0 4.0 ZrO₂ — 5.0 — TiO₂ 7.5 2.5 4.0 Sb₂O₃ — 0.5 0.5 As₂O₃ 0.9 — — F —— — Fe₂O₃ SnO₂ = 1.0 MoO₃ = 0.5 CeO = 1.0 Nucleation temperature (° C.)650 750 670 Crystallization temperature 940 1050 1050 (° C.) Predominantcrystal phases cordierite cordierite cordierite and grain diameter 0.3μm 0.3 μm 0.3 μm β-quartz β-quartz β-quartz 0.1 μm 0.1 μm 0.1 μm Young'smodulus (GPa) 121 143 122 Specific gravity (g/cc) 2.50 2.90 2.70 Young'smodulus (GPa)/specific 48.4 49.3 45.2 gravity Surface roughness Ra (Å) 46 8 Maximum surface roughness 48 78 76 Rmax (Å) Coefficient of thermalexpansion 48 36 41 (× 10⁻⁷/° C.) (−50° C. to +70° C.)

TABLE 5 Examples 13 14 15 SiO₂ 55.5 50.0 56.2 MgO 18.0 13.0 11.0 Al₂O₃10.0 18.0 14.5 P₂O₅ 1.0 3.1 2.0 B₂O₃ 3.5 3.9 3.0 CaO 3.0 1.0 4.0 BaO 1.03.0 4.8 ZrO₂ 5.0 — — TiO₂ 2.5 7.5 4.0 Sb₂O₃ 0.5 0.5 0.5 As₂O₃ — — — F —— — Fe₂O₃ — — — Nucleation temperature (° C.) 750 670 670Crystallization temperature (° C.) 1050 900 1050 Predominant crystalphases cordierite cordierite cordierite and grain diameter 0.3 μm 0.1 μm0.3 μm β-quartz β-quartz β-quartz 0.1 μm 0.1 μm 0.1 μm Young's modulus(GPa) 143 120 122 Specific gravity (g/cc) 2.90 3.10 2.70 Young's modulus(GPa)/specific 49.3 38.7 45.2 gravity Surface roughness Ra (Å) 6 5 8Maximum surface roughness Rmax 78 49 76 (Å) Coefficient of thermalexpansion 36 42 41 (× 10⁻⁷/° C.) (−50° C. to +70° C.)

TABLE 6 Examples 16 17 SiO₂ 55.0 53.5 MgO 15.0 15.0 Al₂O₃ 17.0 18.0 P₂O₅1.0 2.0 B₂O₃ 2.0 — CaO 1.0 2.0 BaO 4.0 2.0 ZrO₂ — — TiO₂ 4.5 7.0 Sb₂O₃0.5 — As₂O₃ — 0.5 F — — Fe₂O₃ — — Nucleation temperature (° C.) 700 700Crystallization temperature (° C.) 950 970 Predominant crystal phasescordiente cordierite and grain diameter 0.3 μm 0.3 μm β-quartz β-quartz0.1 μm 0.1 μm Young's modulus (GPa) 128 134 Specific gravity (g/cc) 2.802.77 Young's modulus (GPa)/specific gravity 44.4 48.4 Surface roughnessRa (Å) 7 6 Maximum surface roughness Rmax (Å) 68 61 Coefficient ofthermal expansion 37 30 (× 10⁻⁷/° C.) (−50° C. to +70° C.)

TABLE 7 Comparative Example 1 2 3 SiO₂ 62.0 78.5 43.0 MgO — — — Al₂O₃16.0 4.4 26.8 P₂O₅ — 2.0 — ZnO — — 23.0 Li₂O 7.0 12.5 — Other alkaliingredients Na₂O 9.0 K₂O 2.8 K₂O 2.4 ZrO₂ 4.0 — — TiO₂ — — — Sb₂O₃ 0.50.2 — As₂O₃ — — — SnO₂ — — — MoO₃ — — — CeO — — — Other ingredient Ga₂O₃4.8 Nucleation temperature (° C.) — 450 800 Crystallization temperature(° C.) — 850 950 Predominant crystal phases — lithium Spinel and graindiameter — disilicate crystal — 0.10 μm 0.10 μm — α-cristobalite — —0.30 μm — Young's modulus (GPa) 82 92 110.5 Specific gravity (g/cc) 2.542.51 3.24 Young's modulus (GPa)/specific 32.3 36.0 34.1 gravity Surfaceroughness Ra (Å) 8 11 65 Maximum surface roughness Rmax 86 140 679 (Å)Coefficient of thermal expansion 70 61 53 (× 10⁻⁷/° C.) (−50° C. to +70°C.)

For manufacturing the glass-ceramic substrate of the above describedexamples, materials including oxides, carbonates and nitrates are mixedand molten in a conventional melting apparatus at a temperature withinthe range from about 1350° C. to about 1450° C. The molten glass isstirred to homogenize it and thereafter formed into a disk shape andannealed to provide a formed glass. Then, this formed glass is subjectedto heat treatment to produce the crystal nucleus under a temperaturewithin the range from 650° C. to 750° C. for about one to twelve hoursand then is subjected to a further heat treatment for crystallizationunder a temperature within the range from 750° C. to 1050° C. for aboutone to twelve hours to produce the desired glass-ceramic. Then, thisglass-ceramic is lapped with lapping grains having average graindiameter ranging from 5 μm to 30 μm for about 10 minutes to 60 minutesand then is finally polished with cerium oxide having average graindiameter ranging from 0.5 μm to 2 μm for about 30 minutes to 60 minutes.

As shown in Tables 1 to 7, the glass-ceramic of the present invention isdifferent in its predominant crystal phase from the Comparative Examplesof prior art alimino-silicate chemically tempered glass, Li₂O—SiO₂glass-ceramics and SiO₂—Al₂O₃—MgO—ZnO—TiO₂ glass-ceramics. As regardsYoung's modulus and specific gravity, the glass-ceramic of the presentinvention has higher rigidity or lower specific gravity than thealumino-silicate chemically tempered glass and the Li₂O—SiO₂glass-ceramics. The SiO₂—Al₂O₃—MgO—ZnO—TiO₂ glass-ceramics ofComparative Example No. 3 which is of a relatively high rigidity and lowspecific gravity is so hard that a desired surface roughness cannot beobtained. In contrast, the glass-ceramics of the present invention hasexcellent processability and desired smoothness. Moreover, theglass-ceramic of the present invention is free from such defects asanisotropic crystals, foreign matters and impurities and has a fine andhomogeneous texture and sufficient chemical durability against rinsingor etching with chemicals or water.

As described above, according to the invention, there is provided aglass-ceramic substrate for a magnetic information storage medium whichhas eliminated the disadvantages of the prior art materials and hasexcellent melting property, resistance to devitrification andprocessability, has sufficient smoothness for coping with the contactrecording with a high recording density and has also high Young'smodulus and low specific gravity characteristics capable of copingproperly with driving of the disk with high speed rotation. According tothe invention, there are also provided a method for manufacturing thisglass-ceramic and a magnetic information storage disk having a film of amagnetic medium formed on the glass-ceramic substrate.

What is claimed is:
 1. A high rigidity glass-ceramic substrate for amagnetic information storage medium having a ratio of Young's modulus tospecific gravity within a range from 37 to 63, which comprises Al₂O₃within a range from 10% to less than 20%, wherein the predominantcrystal phases consist of (1) cordierite or cordierite solid solutionand (2) one or more crystals selected from the group consisting ofSpinel crystal, Spinel crystal solid solution, enstatite, enstatitesolid solution, β-quartz and β-quartz solid solution.
 2. A high rigidityglass-ceramic substrate as defined in claim 1 which is substantiallyfree of Na₂O, K₂O, Li₂O and PbO.
 3. A high rigidity glass-ceramicsubstrate as defined in claim 1 wherein a crystal grain diameter of therespective predominant crystal phase is within a range from 0.05 μm to0.30 μm.
 4. A high rigidity glass-ceramic substrate as defined in claim1 wherein coefficient of thermal expansion within a range from −50° C.to +70° C. is within a range from 30×10⁻⁷/° C. to 50×10⁻⁷/° C.
 5. A highrigidity glass-ceramic substrate as defined in claim 1 wherein thesurface of the substrate after polishing has surface roughness (Ra) of 3to 9 Å and maximum surface roughness (Rmax) of 100 Å or below.
 6. A highrigidity glass-ceramic substrate as defined in claim 1 provided by heattreating a base glass having a composition expressed on the basis ofcomposition of oxides which consists in weight percent of: SiO₂  40-60%MgO  10-20% Al₂O₃  10-less than 20% P₂O₅   0-4% B₂O₃   0-4% CaO 0.5-4%BaO   0-5% ZrO₂   0-5% TiO₂ 2.5-8% Sb₂O₃   0-1% As₂O₃   0-1% F   0-3%SnO₂   0-5% MoO₃   0-3% CeO   0-5% Fe₂O₃   0-5%.


7. A high rigidity glass-ceramic substrate for a magnetic informationstorage medium as defined in claim 6 provided by heat treating the baseglass at a nucleation temperature within a range from 650° C. to 750° C.and at a crystallization temperature within a range from 750° C. to1050° C.
 8. A magnetic information storage disk having a film of amagnetic medium formed on said high rigidity glass-ceramic substrate asdefined in claim 1.